The present invention relates to a mechanical acceleration sensor for activating a gas generator of an inflator in an air bag system or a pretensioner in a seat belt system.
Vehicles, for example, automobiles, are often provided with an air bag system or a seat belt system. For an inflator in the air bag system or a pretensioner in the seat belt system, a gas generator is employed as a device for generating a driving medium for the inflator or the pretensioner. Such a gas generator is activated by an acceleration sensor. Acceleration sensors used for this purpose include two types, namely, a sensor that outputs an electric signal and a mechanical sensor that outputs a mechanical displacement of a member. Electrical sensors are generally superior insofar as the acceleration pulse discriminating ability is concerned, and hence advantageous in that an erroneous operation is unlikely to occur. On the other hand, electrical sensors require a power supply and electrical connections. Therefore, the installation location is somewhat limited. In contrast, mechanical acceleration sensors may be inferior to electrical sensors in acceleration pulse discriminating capability, but they are less costly and need no electric supply or electrical connections. Therefore, mechanical acceleration sensors are advantageous in that they can be used without limitation on the installation location. Under these circumstances, various mechanical acceleration sensors have been proposed and are known.
There is one form of such mechanical sensor in which when a predetermined level of acceleration acts on the vehicle, a firing pin of the sensor is percussively actuated to activate the inflator of the air bag system. This type of mechanical sensor has a weight, a latch lever, and a firing pin, which are movably supported in a sensor casing. The firing pin, which is spring-loaded, is released from the restraint by the latch lever by the inertial movement of the weight relative to the casing, thereby allowing the firing pin to perform a percussive firing action (for example, see Japanese Utility Model Application Laid-Open (KOKAI) No. 2-32464 (1990), the subject matter of which is to make the position of the weight relative to the latch lever adjustable in the above-described arrangement).
In the above-described conventional mechanical sensor, the weight is movably supported in the casing relatively loosely, and hence the behavior of the weight is likely to change, depending upon the position in which the sensor is oriented in a given installation. Accordingly, inaccurate operation due to a particular orientation is always a possibility. In addition, since the space for the operation of the latch lever and the space for the movement of the weight must be provided separately from each other to avoid interference between these two components, the overall size and weight of the sensor become large in comparison to the weight of the weight member.
Such a problem might be solved by allowing the weight to be closely supported in the casing and preventing rotation and rolling of the weight except for movement thereof in the direction of inertia, thereby avoiding interference between the operation of the latch lever and the inertial movement of the weight, and thus enabling weight and lever to share the same space for their respective operations. However, a guide device used for the weight must be formed with high accuracy in order to employ the above-described concept while ensuring the high reliability required for the proper functioning of the weight. To meet the requirements, it is required not only to increase the machining accuracy of each individual constituent element of the sensor, including the casing, the weight, etc., but also to ensure a high assembling accuracy.